Quantifying the association between pregnancy exposure to biomass-attributable PM2.5 and the risk of preterm birth and stillbirth: A case-control study in Sydney, Australia for 2010–2020

Biomass combustion, including wildfires and residential wood burning, is a significant source of particulate matter (PM_2.5) in Australia, with potentially distinct health effects due to its unique chemical composition. This study aimed to quantify the association between exposure to ambient biomass-attributable PM_2.5 and the risk of preterm birth and stillbirth across pregnancy windows in Sydney, Australia, from 2010 to 2020. We conducted case-control studies nested within a cohort of 578,391 singleton pregnancies, including 29,954 preterm births and 2,928 stillbirths. Controls were randomly selected using risk-set sampling. Daily all-source PM_2.5 estimates at a 5 km resolution were obtained from a previous study. Days exceeding the 95th percentile of all-source PM_2.5 at statistical area level 4 without significant dust storm pollution were classified as biomass-affected days. For these days, biomass-attributable PM_2.5 was estimated using the remainder component from a seasonal trend decomposition, with the seasonal and trend components representing nonbiomass-attributable PM_2.5. Conditional logistic regressions were used to analyze associations between biomass-attributable PM_2.5 exposure and outcomes, adjusting for area-level socioeconomic factors, temperature, humidity, and temporal and seasonal trends. The odds ratio for preterm birth per interquartile range increase in biomass-attributable PM_2.5 was 1.002 (95% CI = 0.997, 1.007) for the entire pregnancy average exposure, with similar null results across trimesters. For stillbirth, the odds ratio was 1.002 (95% CI = 0.985, 1.019) for the entire pregnancy average exposure, with comparable null findings across trimesters. These results suggest that in Sydney, biomass-attributable PM_2.5 exposure during pregnancy may not increase the risk of preterm births or stillbirths.